Question

−13π/5 find the reference angle

Answer 1

I would convert to degrees first seems to make it easier.
$\frac{-13 \pi }{5}$×$\frac{180}{ \pi }$
= -468
by adding 360 twice we get the coterminal angle 252

252 is in the 3rd quadrant so 180 is the closest angle on the x-axis
ref angle = 252 - 180 = 72 

If necessary convert back to radians... $72( \frac{ \pi }{180} )= \frac{2 \pi }{5}$